1. Field of the Invention
The present invention relates to a site diversity method for use with digital satellite broadcast and to a digital satellite broadcast receiver based upon site diversity.
2. Description of the Related Art
In digital satellite broadcast, if it is anticipated that the space near at one up-link broadcast station under operation cannot establish a communication line because of rain or the like, the up-link broadcast station is exchanged to another up-link broadcast station. This operation change is called a site diversity method. With this site diversity method, it is desired to suppress the influence of exchanging the up-link broadcast station, such as interception of video and audio signals, as much as possible.
In this specification, an operation exchange from one up-link broadcast station to another up-link broadcast station is called “site exchange” for the purposes of convenience.
For the site diversity method for digital satellite broadcast, it has been proposed that the site exchange is performed in a super frame which is announced and designated in advance. The reason for this is as follows. In order to suppress the influence of site exchange, such as interception of video and audio signals, as much as possible, freeze and mute of a digital satellite broadcast receiver are made not conspicuous during site exchange as much as possible.
Specifically, as shown in FIGS. 4A and 4B, an extension field (first to fourth bits) in the transmission/reception control information in TMCC information is utilized. The fourth bit is set to “1” which indicates a designation of a super frame for executing the site diversity. After the bit is set to “1”, the site diversity is performed in the N-th (N=16) super frame after the super frame with the fourth bit of “1”. After the site diversity is performed, in the M-th (M=0) super frame after the execution of the site diversity, the fourth bit thereof is set to “0”.
In addition, the third bit of the extension field is set to “1” to designate up-link from the main station, whereas the second bit of the extension field is set to “1” to designate up-link from the subsidiary station.
In order to be in conformity with the site diversity method, as shown in FIG. 5, in a demodulator unit of a digital satellite broadcast receiver, a phase error voltage corresponding to a reception phase error is obtained from a detected baseband signal by referring to a carrier reproduction phase error table 21. The phase error voltage is supplied to a carrier filter 22 made of a digital filter. An output of the carrier filter 22 is supplied to an AFC circuit 23 whereat the output of the carrier filter 22 is cumulatively added. In accordance with a cumulative addition value, the oscillation frequency of a numerical control oscillator 24 is controlled. Outputs of the numerical control oscillator 24 are supplied to a calculation circuit 26 as reproduction carriers. The reproduction carriers and baseband signals I and Q input to the calculation circuit 26 are multiplied to detect baseband signals I and Q.
The band widths of the detection outputs from the calculation circuit 26 are limited by a roll-off filter 27. A frame sync detector circuit 31 detects a frame sync pattern from the band-limited detection outputs, and derives TMCC information. The derived TMCC information is supplied to a TMCC decoder 32 which decodes it. The decoded TMCC information is supplied to a site diversity control circuit 33 whereat it is checked from the fourth bit in the extension field whether or not an execution of site diversity is designated or not.
If it is detected that an execution of site diversity is designated, the site diversity control circuit 33 counts down the N-value each time a super frame is detected. When the N-value becomes 0, i.e., when the super frame of N=0 is detected, an execution designation signal is supplied to the frame sync detector circuit 31. Upon reception of this execution designation signal, the frame sync detector circuit 31 continues to detect the frame sync pattern W1 placed before the TMCC information field. When the frame sync pattern W1 is detected, synchronization is again performed by using the frame sync pattern W1.
With the site diversity method conventionally proposed, however, site exchange is performed at some position in the designated super frame and it is not clear at which position in the designated super frame the site exchange is performed.
It is conceivable that there are a deviation of carrier frequencies, a shift of clock phases and the like between a main station and a subsidiary station at the time of site exchange. It is also conceivable that there is a deviation of a frame period before and after site exchange, e.g., about 5 msec. With these deviation and shift, images are disturbed or voices are intercepted.
This conventional site exchange is performed at some position in the super frame, and at this exchange instant the carrier frequency, clock phase, frame period and the like are deviated and shifted. It is therefore necessary to release a system lock once on the side of a digital satellite broadcast receiver and perform synchronization again. Since it is indefinite that site exchange is performed at what position in a super frame, system lock is performed in the next super frame at the fastest to perform synchronization again. This period is a shortest period required for re-synchronization.
Therefore, data in the super frame at which the site exchange is performed cannot be reproduced. For example, video data in a digital satellite broadcast receiver has error so that images are disturbed and voices are intercepted.